Walsh function mixer and tone detector

ABSTRACT

A mixer stage having particular application to digital detectors. The mixer stage includes first and second sets of transmission gates and a Walsh function generator. The first set of transmission gates is driven by sine function coefficient signals from the Walsh function generator so as to open and close individual gates of the set so as to effectively multiply an input signal by the sine of a tone reference frequency to be tone detected. Similarly, the second set of transmission gates is driven by cosine coefficient signals from the Walsh function generator so as to open and close individual gates of the second set to effectively multiply the input signal by the cosine of the reference frequency to be tone detected. To form a complete digital tone detector, the sine and cosine multiplied outputs of the mixer stage are each filtered and squared and are then summed and further filtered to provide a &#34;detect&#34; signal when the input signal includes the reference frequency.

BACKGROUND OF THE INVENTION

This invention relates in general to tone detectors. More specifically,the invention relates to tone detectors such as, for example those ofthe type used in radio receivers to detect a particular tone modulatedon a received carrier wave such as used in the addressing of pagingradios and the like. More specifically, the invention provides a mixerstage for multiplying a received and carrier demodulated input signal bya reference frequency to determine whether there are any components ofthe reference frequency in the input signal.

Various known tone detector arrangements are shown in the followingpatents, the teachings of which are specifically incorporated herein byreference.

U.S. Pat. No. 4,275,271--Soulliard--June 23, 1981

U.S. Pat. No. 4,258,423--Lane, et al--Mar. 24, 1981

U.S. Pat. No. 4,142,177--Davis--Feb. 27, 1979

U.S. Pat. No. 4,047,114--Lane, et al.--Sept. 6, 1977

U.S. Pat. No. 4,021,653--Sharp et al.--May 3, 1977

U.S. Pat. No. 3,962,645--Stewart--June 8, 1976

The above list is intended only to be exemplary of the prior art relatedto tone detectors and is not intended to be an exhaustive list.

It is known to utilize a sine and/or cosine multiplied input signal todetermine the presence or absence of a frequency component of areference signal in the input signal. Specifically, sine and cosinemixers are used in digital tone detectors for detecting the presence orabsence of a specific tone in a signal containing a large number offrequencies. There are many known methods for accomplishing thisobjective including the use of complex filtering arrangements.

Typically, an input signal to be tone detected is mixed with a pure sinewave signal of the tone frequency to be detected (also referred to asthe "reference frequency"). The mixer, in effect, multiplies the inputsignal by the reference frequency to produce a DC component in the mixed(multiplied) output whenever a frequency component of the input signalis equal to the reference frequency to be detected. The mixer output islow pass filtered and subjected to the transfer function of a square lawdevice to develop a signal proportional to the power of the mixedsignal. A popular mixer used in this type of tone detector is afour-quadrant multiplier, such as for example an MC 1595L integratedcircuit with appropriate offset adjustment networks. A problem with thisknown approach is that the tone detector is relatively complex andexpensive.

This invention is directed to a mixer stage that can replace the knownfour-quadrant mixer in a tone detector and to the tone detector circuitincluding the mixer stage. In essence, a known mixer stage is replacedby a set of transmission gates driven by a Walsh function generatorproviding either sine or cosine Walsh function coefficient signals tothe gates causing them to open and close. The gates effectively multiplyan input signal to be tone detected by the sine or cosine functiondefined by the Walsh function coefficient signals.

A Walsh function generator works by logically breaking down an inputsignal such as for example a square wave or other signal withrecognizable edges into a group of several periodic pulse trains knownas Walsh functons. Certain ones of the Walsh functions may be combinedin a Walsh weighting and summing network to form a stair-step sine wave.The sine wave frequency is directly related to the frequency of a squarewave input to the Walsh function mixer. Similarly, a stair-step cosinewave can be constructed using the remaining Walsh functions. A Walshweighting and summing network including properly valued resistorsseparately attenuate each of the Walsh functions to a desired level.These attenuated Walsh functions are then summed to create the desiredwave form. In this particular circuit, the Walsh function outputs of theWalsh function generator are used to open and close individual gates ofthe transmission gates sets. The sine coefficient signals from the Walshfunction generator are used to open and close the gates of the "sine"set of transmission gates and the cosine coefficient signals from theWalsh function generator are used to open and close the gates of the"cosine" set of transmission gates.

Walsh function generators per se are known and used in otherapplications. See for example the following documents, the contents ofwhich are hereby incorporated by reference:

U.S. Pat. No. 4,052,565--Baxter et al (Oct. 4, 1977)

U.S. Pat. No. 4,047,009--Challen (Sept. 6, 1977) "Walsh Functions: ADigital Fourier Series" by Benjamin Franklin Jacoby, Ph.D., InformationConversion Devices Co., 88 W. Frankfurt St., Columbus, OH 43206.

The Baxter '565 patent shows a Walsh function generator used in adigital speech scrambler. The Challen '009 patent teaches how to use aWalsh function generator in a digital tone generator for a radiocontrolled squelch system. The Jacoby article provides a discussion ofthe mathematical basis for synthesizing waveforms using Walsh functions.

SUMMARY OF THE INVENTION

The present invention provides a Walsh function mixer stage and a tonedetector incorporating the Walsh function mixer stage for use in varioustypes of electronic equipment such as, for example, radios. The tonedetector is intended for use in determining the presence or absence of aspecific tone frequency component in a received and carrier demodulatedsignal possibly containing a large number of frequencies.

An input signal such as a received and carrier demodulated radio signalto be tone detected is coupled to a first section (sine section) of themixer stage including a sine mixer which multiples it with a sine waveof the desired tone frequency. The output of the mixer has a DCcomponent whenever the reference is present in the input signal. Themixer output is coupled through a low pass filter to a square lawdevice. A tone input of the desired frequency to the mixer will resultin an output signal from the square law device. This output will be afunction of the power of the input signal and the relative phase anglebetween the frequency component being detected and the reference signalmixed with the input signal being tone detected. It is possible thatdespite the presence of a component of the reference frequency in theinput signal, the square law device may provide a "zero" output if therelative phase angle happens to be 90° or 270°.

Therefore, a second section (cosine section) of the mixer stageincluding a cosine mixer, low pass filter and square law device isprovided. In this second section, the input signal is mixed with acosine of the reference frequency rather than with the sine of thereference frequency. The outputs of the square law devices of the sineand cosine sections are summed to form the tone detector output.

Each of the sine and cosine mixers are formed by a set of transmissiongates each having an input signal for receiving the signal to be tonedetected. The transmission gates are driven to open and close by theoutput signals of a Walsh function generator for synthesizing a sine orcosine function. In essence, the Walsh function generator provides thesine and cosine terms of the reference frequency to be tone detected.These sine and cosine terms are used to effectively create a sine andcosine signal for multiplying the input signal. By using a Walshfunction generator, the tone detector becomes digitally programmable andthe multiplied signals are harmonically pure. It becomes easy togenerate in-phase sine and cosine outputs of the reference frequency.

In essence, the present invention provides a mixer stage for multiplyingan input signal by the sine and cosine of a reference frequency,comprising:

(a) input means for receiving the input signal;

(b) a first set of transmission gates, each gate having an input forreceiving the input signal, an output and a control input for receivinga control signal for causing the gate to pass the input signal from thegate input to its output or block the input signal so that it does notpass to the output;

(c) first means associated with said first set of transmission gates forcombining the signals at the outputs thereof into a first mixer sectionoutput providing a signal defined by the input signal multiplied by thesine of the reference frequency;

(d) first means associated with said first set of transmission gates forweighting the contributions of the signals from each transmission gateto said first combining means;

(e) a second set of transmission gates, each gate having an input forreceiving the input signal, an output, and a control input for receivinga control signal causing the gate to pass the input signal from the gateinput to its output or block the input signal so that it does not passto the output;

(f) second means associated with the second set of transmission gatesfor combining signals at the respective outputs thereof into a secondmixer section output providing a signal defined by the input signalmultiplied by the cosine of the reference frequency;

(g) second means associated with the second set of transmission gatesfor weighting the contributions of the signals from each transmissiongate to said second combining means;

(h) a reference frequency source;

(i) Walsh function generating means having an input coupled to saidreference frequency source, a first set of outputs for providing signalsrepresenting a sine wave synthesis of reference frequency from Walshfunction pulse trains, the first set of outputs being coupled to saidcontrol inputs of the first set of transmission gates, and a second setof outputs for providing signals representing a cosine synthesis of thereference frequency from Walsh function pulse trains, the second set ofoutputs being coupled to said control inputs of said second set oftransmission gates.

The invention also provides a digital tone detector for determining thepresence or absence of a predetermined reference frequency component inan input signal comprising:

input means for receiving said input signal;

a first Walsh function mixer having a signal input coupled to said inputmeans, a set of control inputs and an output for providing an outputsignal defined by said input signal multiplied by the sine of saidreference frequency;

a second Walsh function mixer having a signal input coupled to saidinput means, a set of control inputs and an output for providing anoutput signal defined by said input signal multiplied by the cosine ofsaid reference frequency;

a reference frequency source;

a Walsh function generator having an input coupled to said referencefrequency source for generating, at a first set of outputs, signalsrepresenting a sine wave synthesis of said reference frequency, saidfirst set of outputs being coupled to said control inputs of said firstWalsh function mixer and for generating, at a second set of outputs,signals representing a cosine wave synthesis of said referencefrequency, said second set of outputs being coupled to said controlinputs of said second Walsh function mixer;

a first low pass filter coupled to said output of said Walsh functionmixer;

a first squaring circuit coupled to said first low pass filter;

a second low pass filter coupled to said output of said second Walshfunction mixer;

a second squaring circuit coupled to said second low pass filter;

means for combining signals from said first and second squaring circuitsto provide an output signal wherever a component of said referencefrequency is present in said input signal.

BRIEF DESCRIPTION OF THE DRAWINGS

An exemplary embodiment of the present invention constituting thepresently preferred best mode for carrying out the invention will bedescribed in detail with respect to the drawings wherein:

FIG. 1 is a block diagram of a sine mixer section of the mixer stagetone detector circuit according to the present invention;

FIG. 2 is a cosine mixer section of the mixer stage and detector circuitaccording to the present invention;

FIG. 3 is a block diagram of the complete tone detector circuitaccording to the present invention;

FIG. 4 is a graphical representation of a stairstepped sine wavesynthesized by a Walsh function generator;

FIG. 5 is a Walsh weighting and summing network used to combine theWalsh function outputs to synthesize a sine wave;

FIG. 6 is a block diagram of the sine and cosine mixer stages accordingto the present invention;

FIG. 7 is a schematic diagram of the mixer sections shown in FIG. 6 and

FIG. 8 is a block diagram of the digital tone detector according to thepresent invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENT

Referring now to FIG. 1 there is shown in block diagram the operativeportions of a first section (sine section) of the tone detectoraccording to the present invention. An input signal desired to be tonedetected is coupled to an input 10 of a mixer 12 for multiplying theinput signal by a sine wave of the desired detection frequency appliedat an input 14 of mixer 12. An output 16 of mixer 12 will include a d.c.component whenever the desired tone is present in the input signal. Theoutput of mixer 12 is filtered by a low pass filter 18 and then coupledthrough a square law device 20 to produce a first section output signal.

Assume that the input signal to be tone detected includes a componentdefined by Equation (1) below and that the reference frequency to betone detected is w₁

    V SIN (w.sub.2 t+φ)                                    (1)

If the mixer is a linear multiplier for multiplying the input signal bySIN w₁ t, its output will be as shown in Equation (2) which can bemathematically manipulated to form Equation (3). ##EQU1##

If the frequency of the input signal is equal to that of the desiredtone then w₂ -w₁ =O and the first term of the mixer output becomes thatshown in Equation (4), a d.c. value. ##EQU2##

Low pass filter 18 coupled to the output of mixer 12 designed to passonly those frequencies which are at or slightly above d.c. Therefore,low pass filter 18 will provide an output only when w₂ is equal to orwithin a few radian/second of w₁.

Square law device 20 is coupled to the output of the low pass filter toobtain an output proportional to the power of the input signal. Thedetection of signal power rather than of signal level results in apositive output of the square law device. Thus, a tone input of thedesired frequency (within a few radian/second) results in an output fromthe square law device equal to that shown in equation (5) below and atone input of any other frequency results in an output of zero ##EQU3##

The output of square law device 20 is a function of the relative phaseangle φ of the input signal. Therefore, for certain values of φ, thedetector output could be zero even with the reference tone frequency ispresent at the input. Because of this effect, it is appropriate to add asecond section to the detector as shown in FIG. 2.

Referring now to FIG. 2, the input signal represented by Equation (1) iscoupled to an input 22 of a cosine mixer 24. In mixer 24, the inputsignal is multiplied by the cosine of the reference frequency coupled toan input 26 of mixer 24 to form a signal at an output 28 of the mixerrepresented by in Equation (6) below.

    V SIN (w.sub.2 t+φ)·COS w,t                   (6)

The output signal is filtered by a low pass filter 30 and then coupledthrough a square law device 32 in the same manner that the sinemultiplied signal is processed as shown in FIG. 1. The output of squarelaw device 32 in the cosine section would be defined by Equation (7)below. ##EQU4##

Referring now to FIG. 3 there is shown a block diagram of a completetone detector including both the sine and cosine sections shown in FIGS.1 and 2. As shown, the input signal is coupled to input 10 of mixerstage 12 and to input 22 of mixer stage 24. The outputs of square lawdevices 20 and 32 are summed by a summer 34 to form a tone detectoroutput signal. The output of summer 34 can be represented by Equations(8) (9) and (10) below: ##EQU5## The detector output signal V² /4 is aDC value which may be used to indicate the presence of the desired tonein the input

Thus far, mixers 12 and 24 are shown only as general blocks. Noreference has been made to the particular method used to generate thesine and cosine terms of the reference frequency. The referencefrequency could be generated by any number of known methods includingcrystals, phase locked loops or filtered square waves. However,applicants have recognized the advantage of utilizing a Walsh functiongenerator for forming the sine and cosine component coupled to inputs 14and 26 respectively of mixers 12 and 24.

The Walsh function generator offers several advantages. First, the useof a Walsh function generator allows for digital programmability.Secondly, it provides a signal with low harmonic content in its outputwave form and thirdly, it allows for an easy method for generatingcoherent sine and cosine outputs of the same frequency. A Walsh functiongenerator works by logically breaking down an input square wave into agroup of several periodic pulse trains known as Walsh functions. Certainones of these Walsh functions may be combined in a Walsh weighting andsumming network to form a stair-step sine wave as shown in FIG. 4. Thesine wave frequency is directly related to the frequency of the inputsquare wave. Similarly, a stair-step cosine wave may be constructedusing the remaining Walsh functions.

Referring now to FIG. 5 there is shown a weighting and summing network.The weighting and summing network includes a plurality of appropriatelyvalued resistors which separately attenuate each of the Walsh functionsto a desired level and a summing node at which the properly weightedWalsh functions are combined to create the desired sine or cosine waveform. The summing network shown in FIG. 5 provides the sinestair-stepped wave shown in FIG. 4 by utilizing Walsh functions SAL 1 .. . SAL N. A true sine wave can be extracted from the stair-stepped sinewave using a low pass filter. The following description is related tothe mixers 12 and 22.

Normally, mixers 12 and 22 would be implemented using four quadrantmultipliers. However, as a substitute for four quadrant multipliers,each mixer is implemented by a group of transmission gates.

Referring now to FIG. 6 there is shown a mixer stage according to thepresent invention incorporating a Walsh function generator 50. Walshfunction generator 50 generates at a first set of outputs sine Walshfunctions SAL 1 . . . SAL N and at a second set of outputs cosine Walshfunctions CAL 1 . . . CAL N. The input to Walsh function generator 50 isthe reference frequency (REF. FREQ.) to be tone detected. The sinecoefficient outputs of Walsh function generator 50 are coupled tocontrol inputs of a first set of tranmission gates 56 including gates56-1 . . . 56-N. In essence, input 59 of transmission gates set 56corresponds to input 10 of mixer 12 shown in FIGS. 1 and 3. Eachtransmission gate 56-1 . . . 56-N includes an input coupled to input 59of the set 56 of transmisson gates, a control signal input coupled toone of the sine coefficient signals from Walsh function generator 50 andan output. The outputs are combined by a resistor summing networkincluding resistors 60-1 . . . 60-N. These resistors couple signals fromtheir respective transmission gates to a common summing node 61 formingan output signal of the sine mixer section at an output 62 oftransmission gate set 56. Resistors 60-1 . . . 60-N perform the sameweighting function as the illustrative resistor summing network shown inFIG. 5 except that in FIG. 6, weighting occurs after multiplication inthe mixer.

Similarly, the cosine coefficient signals CAL-1 . . . CAL-N from Walshfunction generator 50 are coupled to the control inputs of individualtransmission gates 58-1 . . . 58-N of transmission gate set 58. Each oftransmission gates 58-1 . . . 58-N includes an input coupled to an input64 of transmission gate set 58. Each transmission gate 58-1 . . . 58-Nalso includes an output coupled via a resistor 66-1 . . . 66-N,respectively to a common node 68 forming an output signal coupled to anoutput 70 of transmission set 58.

Referring now to FIG. 7 there is shown a schematic diagram of a specificcircuit configuration of the mixer arrangement shown in general blockform in FIG. 6. Dotted lines define the basic block elements as shown inFIG. 6 and use corresponding reference numerals for sine mixer 56,cosine mixer 58 and Walsh function generator 50.

Walsh function generator 50 receives a signal wt at an input 100thereof. The signal is coupled to the clock input of a 12 stage ripplecounter 102 preferably constituted by a 4040 integrated circuit. Ripplecounter 102 includes Q3, Q4, Q5 and Q6 outputs providing divide by 8,divide by 16, divide by 32 and divide by 64 outputs, respectively. As analternative, the outputs could be divide by 2, divide by 4, divide by 8and divide by 16 or any successively declining power of 2. The ripplecounter 102 outputs are coupled to the inputs of a logic circuitincluding gates 202-224 and a resistor network including resistors 230,232 and 234 to provide the sine and cosine coefficient signals definingthe sine and cosine Walsh functions. Gates 202, 204, 206, 208, 210, 218,220 and 224, are preferably constituted by qaud packaged gates such as,for example 4030 gates. Of course, other types of logic circurts couldalso be used in place of gates, such as for example, relays. Gates 212,214, 216 and 222 are preferably constituted by 4049 HEX inverterintegrated circuits. It will be appreciated by those of ordinary skillin the art that one could choose to generate a different number of Walshfunctions than as shown in the FIG. 7 embodiment. In this embodiment theQ3, Q4, Q5 and Q6 outputs of ripple counter are used but a different setof outputs could have been selected. The number of Walsh functionsgenerated determines the relationship between the reference clockfrequency and the frequency of the tone to be detected. For example, iffive (5) Walsh functions were to be used, one would clock at 2₅ timesthe tone frequency to be detected.

The sine coefficient signals are labeled SAL1, SAL3, SAL5 and SAL7. Thecosine coefficient signals are labeled CAL1, CAL3, CAL5 and CAL7. Thesine coefficient signals are coupled to the control inputs oftransmission gates (T.G.) 106, 108, 110 and 112 of sine mixer 56. Thecosine coefficient signals are coupled to the control input oftransmission gates 120, 122, 124 and 126 of cosine mixer 58. Thetransmission gates are preferably constituted by CMOS quad-bilateralswitches using 4066 integrated circuits. The outputs of transmissiongates 106 . . . 112 of sine mixer 56 are summed with a resistor summingnetwork including resistors 130, 132, 134 and 136. The summed output iscoupled through an amplifier stage 140 to provide a sine multipliedoutput of sine mixer 56 at an output terminal 142 thereof.

Similarly, the outputs of transmission gates (T.G.) 120, 122, 124 and126 in cosine mixer 58 are summed with a resistor summing networkincluding resistors 150, 152 and 154 and 156. The combined outputs ofthe transmission gates are coupled through an amplifier stage 160 toprovide a cosine multiplied output at an output terminal 170 of themixer stage. The input signal to be mixed and tone detected is coupledto an input 180 of the mixer stage and is amplified by an amplifierstage 182 before it is coupled to the input of each of the transmissiongates in the sine mixer 56 and cosine mixer 58.

Referring now to FIG. 8 there is shown a block diagram of a tonedetector incorporating the mixer stage shown in block diagram in FIG. 6and schematically in FIG. 7. Walsh function generator 50, sine mixer 56and cosine mixer 58 are represented by the same reference numerals usedin FIGS. 6 and 7. An input signal to be tone detected is filtered by ahigh pass filter 300 before being coupled to input 59 of sine mixer 56and input 64 of cosine mixer 58. Walsh function generator 50, sine mixer56 and cosine mixer 58 were described in detail with reference to FIGS.6 and 7 and therefore will not be further described. The input to Walshfunction generator 50 is through a programmable counter 302 havingprogramming inputs 304 for determining the input reference frequency toWalsh function generator 50. A stable reference frequency is applied toan input 306 of programmable counter 302.

The output of sine mixer 56 is coupled through low pass filter 18 (alsoshown in FIG. 3). The output of low pass filter 18 is coupled throughsquare law device 20. The output of cosine mixer 58 is coupled throughlow pass filter 30 and squaring circuit 32. The outputs of squaringcircuits 20 and 32 are summed by summer 34 and low pass filtered by alow pass filter 310 to provide a tone detector output 312.

While the invention has been described in connection with what ispresently considered to be most practical and preferred embodiments, itis to be understood that the invention is not to be limited to thedisclosed embodiments, but on the contrary, is intended to cover variousmodifications and equivalent arrangements included within the spirit andscope of the appended claims which scope is to be afforded the broadestinterpretation so as to encompass all such modifications and equivalentstructures.

What is claimed:
 1. A mixer stage for multiplying an input signal by thesine and cosine of a reference frequency, comprising:(a) input means forreceiving said input signal; (b) a first set of transmission gates, eachgate having an input means for receiving said input signal, an output,and a control input for receiving a control signal for causing the gateto pass the input signal from the gate input to its output or block theinput signal so that it does not pass to the output; (c) first meansassociated with said first set of transmission gates for combining thesignals at the outputs thereof into a first mixer section outputproviding a signal defined by the input signal multiplied by the sine ofthe reference frequency; (d) first means associated with said first setof transmission gates for weighting the contributions of the signalsfrom each transmission gate to said first combining means; (e) a secondset of transmission gates, each gate having an input means for receivingsaid input signal, an output and a control input for receiving a controlsignal for causing the gate to pass the input signal from the gate inputto its output or block the input signal so that it does not pass to theoutput; (f) second means associated with said second set of transmissiongates for combining signals at the outputs thereof into a second mixersection output providing a signal defined by the input signal multipliedby the cosine of the reference frequency; (g) second means associatedwith said second set of transmission gates for weighting thecontributions of the signals from each transmission gate to said secondcombining means; (h) a reference frequency source; (i) Walsh functiongenerating means, having an input coupled to said reference frequencysource, for generating a first set of outputs representing a sine wavesynthesis of reference frequency from Walsh function pulse trains, saidfirst set of outputs being coupled to said control inputs of said firstset of transmission gates, and for generating a second set of outputsrepresenting a cosine synthesis of said reference frequency from Walshfunction pulse trains, said second set of outputs being coupled to saidcontrol inputs of said second set of transmission gates.
 2. A mixerstage according to claim 1 wherein said first weighting means and firstcombining means together comprise a first summing resistor network, theresistor values determining the weighting of the contributions of eachtransmisson gate of said first set of transmission gates.
 3. A mixerstage according to claim 1 or 2 wherein said second weighting means andsaid first combining means together comprise a second summing resistornetwork, the resistor values determining the weighting of thecontribution of each transmission gate of said second set oftransmission gates.
 4. A mixer stage according to claim 1 whererin saidtransmission gates are CMOS transmission gates.
 5. A mixer stageaccording to claim 1 wherein said Walsh function generating meanscomprises:a ripple counter having a clock input coupled to saidreference frequency source and a set of outputs each output representinga different division of the clock input; and a logic gate network havinginputs coupled to said set of outputs of said ripple counter, aplurality of outputs constituting said first set of outputs of saidWalsh function generating means and a plurality of outputs constitutingsaid second set of outputs of said Walsh function generating means.
 6. Amixer stage according to claim 5 wherein said ripple counter comprises a4040 integrated circuit.
 7. A mixer stage according to claim 1, 5 or 6further including a gain controllable stage coupled between said inputmeans and said first and second sets of transmission gates.
 8. A mixerstage according to claim 5 or 6 wherein said transmission gates comprisequad bilateral switches.
 9. A mixer stage according to claim 8 whereinsaid quad bilateral switches comprise 4066 integrated circuits.
 10. Amixer stage according to claim 1, 5 or 6 further including an amplifierstage coupled to said first combining means.
 11. A mixer stage accordingto claim 1, 5 or 6 further including an amplifier stage coupled to saidsecond combining means.
 12. A digital tone detector for determining thepresence or absence of a predetermined reference frequency component inan input signal comprising:input means for receiving said input signal;a first Walsh function mixer having a signal input coupled to said inputmeans, a set of control inputs and an output for providing an outputsignal defined by said input signal multiplied by the sine of saidreference frequency; a second Walsh function mixer having a signal inputcoupled to said input means, a set of control inputs and an output forproviding an output signal defined by said input signal multiplied bythe cosine of said reference frequency; a reference frequency source; aWalsh function generator having an input coupled to said referencefrequency source for generating, at a first set of outputs, signalsrepresenting a sine wave synthesis of said reference frequency, saidfirst set of outputs being coupled to said control inputs of said firstWalsh function mixer and for generating, at a second set of outputs,signals representing a cosine wave synthess of said reference frequency,said second set of outputs being coupled to said control inputs of saidsecond Walsh function mixer; a first low pass filter coupled to saidoutput of said Walsh function mixer; a first squaring circuit coupled tosaid first low pass filter; a second low pass filter coupled to saidoutput of said second Walsh function mixer; a second squaring circuitcoupled to said second low pass filter; means for combining signals fromsaid first and second squaring circuits to provide an output signalwhenever said input signal includes, as a component thereof, saidreference frequency.
 13. A tone detector according to claim 12 furtherincluding third low pass filter for filtering signals from saidcombining means.
 14. A tone detector according to claim 12 wherein saidfirst Walsh function mixer comprises a first set of transmission gates,each gate having an input coupled to said input means for receiving saidinput signal, an output and a control input for receiving a controlsignal causing the gate to pass the input signal from the gate input toits output or block the input signals so that it does not pass to theoutput, the control signals being the signals at said first set ofoutputs of Walsh function generator.
 15. A tone detector according toclaim 12 wherein said second Walsh function mixer comprises a second setof transmission gates, each gate having an input coupled to said inputmeans for receiving said input signal, an output and a control input forreceiving a control signal causing the gate to pass the input signalfrom the gate input to its output or block the input signals so that itdoes not pass to the output, the control signals being the signals atsaid second set of outputs of said Walsh function generator.
 16. A tonedetector according to claim 14 or 15 wherein said transmission gatescomprise CMOS transmission gates.
 17. A tone detector according to claim14 or 15 wherein said transmission gates comprise quad bilateralswitches.
 18. A tone detector according to claim 17 wherein said quadbilateral switches comprise a 4066 integrated circuit.
 19. A tonedetector according to claim 14 wherein said first Walsh function mixerfurther includes first means associated with said first set oftransmission gates for combining the signals at the outputs thereof toform the output of said first Walsh function mixer.
 20. A detectoraccording to claim 19 further including first means associated with saidfirst set of transmission gates for weighting the contribution of thesignals from each transmission gate to said first combining means.
 21. Atone detector according to claim 15 wherein said second Walsh functionmixer further includes second means associated with said second set oftransmission gates for combining the signals at the output thereof toform the output of said second Walsh function mixer.
 22. A tone detectoraccording to claim 21 further including second means associated withsaid second set of transmission gates for weighting the contribution ofthe signals from each transmission gate to said second combining means.23. A tone detector according to claim 20 wherein said first weightingmeans and said first combining means together comprise a first summingresistor network, the resistor values determining the weighting of thecontribution of each transmission gate of said first set of tranmissiongates.
 24. A tone detector according to claim 22 wherein said secondweighting means and said second combining means together comprise asecond summing resistor network the resistor values determining theweighting of the contribution of each transmission gate of said secondset of transmission gates.
 25. A tone detector according to claim 12wherein said Walsh function generator comprises:a ripple counterintegrated circuit having a clock input coupled to said referencefrequency source and a set of outputs, each output representing adifferent division of the clock input; and a logic gate network havinginputs coupled to said set of outputs of said ripple counter and aplurality of outputs constituting said first set of outputs of saidWalsh function generating means and a plurality of outputs constitutingsaid second set of outputs of said Walsh function generating means. 26.A tone detector according to claim 25 wherein said ripple counterintegrated circuit comprises a 4040 integrated circuit.